Well, he’s a Darwin experimenter ...
What surprises me the most is that the breech didn’t explode: imagine the over pressure from a barrel full of water.
For those who can’t go to the video - the bullet only goes about a foot or two before becoming totally spent and dropping to the bottom.
While a round may not hurt you in the water (Myth Busters did something similar) an explosion will. It was a big deal that the depth charges be set to safe when a ship was going down. Supposedly, if you are in the water and there is a large explosion (I mean really large), the concussive wave passing through the water and eventually your body could blow your innards out your backside. You swim away from the vessel, not to avoid the vortex that might suck you down (This is thought to be a myth too) but to be away in the event of an explosion.
I wonder what the result would be if you shot down into the water from about 8 feet away. Would the bullet travel farther if it had a chance to develop some velocity in the air?
Mythbusters tested this extensively, only with bullets fired from above the water. If I recall correctly, most of the bullets broke up on impact with the water. None penetrated the water with killing speed for more than a few inches.
I once read that the frogmen os WWII said that if they wer more than 3 feet underwater, they were safe from gunfire from the beaches.
IIRC, firing a FMJ 9mm from a Glock underwater can still penetrate plywood at about a dozen feet.